ESSENCE is a 5 year project being carried out on the CTIO
4-m telescope to discover 200 Type Ia supernovae in the
redshift range 0.2 to 0.8, with the purpose of estimating
the equation of state parameter of the universe (w) to
± 10 percent. In each ESSENCE cycle, we observe the
first half of every other night during the dark/grey nights
in October, November, and December. We are presently in the
third year of the project.

Here we present a selection of nine supernovae discovered
and followed by ESSENCE in cycle two and observed by the
Hubble Space Telescope (HST). HST is particularly useful
in defining the decay after maximum for the highest redshift
events in the ESSENCE sample. The preliminary ground-based
photometry is in the R- and I-bands, while the HST
observations with the Advanced Camera for Surveys were taken
in the F625W, F775W, and F850LP filters. The observed
photometry is transformed to the rest frame B- and
V-bands, from which we derive the reddening, absolute
magnitudes at maximum, and luminosity distances. We find
that the HST observations start after maximum but their
high signal-to-noise are critical in defining the light
curve shape for the faint targets at higher redshifts (z >
0.5) discovered with the CTIO 4-m.

The different ground-based and space-based bandpasses
present an excellent test of the systematic errors
introduced in various photometric systems. We combine these
nine objects with those on the current high redshift Hubble
diagram and project cosmological constraints that will be
obtained from the final ESSENCE data set.